The present invention relates generally to workpiece design for press forming operations, and more particularly to a method for selecting suitable workpiece materials utilizing customized strain and bending correlations.
As is generally known, many consumer and-industrial goods are constructed by press forming a relatively pliable workpiece material into the desired product shape. For example, refrigerators, ovens, storage cabinets, beverage containers, tool chests, and automobile body parts are often constructed by press forming aluminum or steel sheets. Likewise, many airplane components, such as the fuselage, chair frames, and structural support members are often constructed by forging or press forming aluminum sheets or blanks.
In many cases, it is critical that the workpiece material will not rupture or yield when it is formed into the desired shape. As such, the workpiece material must be sufficiently ductile, the workpiece shape must be compatible with the types of bends that will be made, and there must be a sufficient quantity of workpiece material so it can be stretched and manipulated into the final shape. In addition, designers often prefer to minimize the size and weight of the workpiece. For instance, a smaller and lighter workpiece can be an important design parameter, such as with airplane and rocket components. Furthermore, a smaller and lighter workpiece can reduce per unit material cost, and may reduce the number of post-forming operations that are required, such as trimming excess material.
While the art of selecting and designing workpiece materials is very old, the process can be very expensive, time consuming, and inaccurate. In some cases, designers may refer to various engineering manuals to evaluate the formability of a particular material. If the product is formed from a common material, substantial information may be available regarding the material strength and ductility. For common materials, general bending correlations may also be available for predicting strain when the workpiece is bent and stretched into typical shapes. This approach can be inaccurate or impossible however, if the finished product is formed from a relatively new or rare material. Moreover, standard strain correlations can be inaccurate if the workpiece bends are different from those used to develop the correlations.
In addition to the above difficulties, complex material properties are often difficult to predict. For example, strain correlations may not be available for predicting the combined strain from multiple bends disposed about the workpiece. Combined strain can be a problem when the strain from the respective bends do not cause fracture when considered in isolation, but their combined strain leads to failure in the workpiece. Similarly, standard engineering manuals often do not have accurate correlations for predicting the tendency for a workpiece to spring back after press forming. Thus, the workpiece final shape may deviate from the intended shape when this tendency is not accounted for.
To address these problems, it is known in the art to select workpiece materials by experimentation. Experimental methods normally involves bending multiple workpiece samples into the desired product shape so that an optimum workpiece design can be determined by trial and error. While this approach can provide accurate information regarding strain properties and the tendency to spring back, it can also be expensive and time consuming. For instance, extensive testing is generally required for new materials or when the workpiece is formed into relatively unique shapes. As such, budgetary constraints may limit the amount of time and money for workpiece testing, and a less than optimum design may be selected for expediency. In addition, even if a near optimum workpiece design can be developed, the test data may not be useful if the product shape is subsequently modified. As a result, testing may have to be repeated when the finished product changes.
In view of the above complexities, a primary object of the present invention is to provide a more accurate method for determining workpiece formability using customized strain and bending correlations for the workpiece material.
Another object of the present invention is to provide a method for predicting the extent of workpiece spring back after press forming.
Yet object of the present invention is to provide a method for determining the compounded strain resulting form multiple bends.
Moreover, another object of the present invention is to provide a method for determining bend angles to predict whether a workpiece may be formed in one step forming operation.
Still another object of the present invention is to provide a method for determining workpiece formability which minimizes the time and expense for workpiece testing.
These and other objects of the present invention will become apparent from the disclosure which now follows.
The present invention is a method for selecting a suitable workpiece having a material composition and a thickness for forming an article. The method includes the steps of selecting a workpiece, obtaining a yield strain for the workpiece material, and determining whether the article has at least one straight bend wherein each straight bend defines a respective straight bend axis. If the article has at least one straight bend, the user inputs a straight bend radius and a straight bend angle for each straight bend, and calculates a straight bend strain across each respective straight bend axis. This calculation can be accomplished utilizing a customized strain correlation for the workpiece material as developed from strain test data of workpiece samples. The straight bend strain for each bend can then be compared to the workpiece yield strain. If the straight bend strain at least equals the material yield strain, the workpiece can be classified unsuitable, and an alternative workpiece can be selected. In this case, the evaluation can be repeated for the alternative workpiece; however, the same strain correlation can be utilized if the same work piece material is selected. If on the other hand, the straight bend strain is less than the material yield strain, the workpiece can be classified suitable, pending the outcome of other workpiece evaluations.
The method also determines whether the article has at least one stretch flange defining a corner axis and a centerline axis. If so, the user inputs a bend angle, a bend radius, a bend arc length, a flange width, and a contour radius for each stretch flange, and calculates a stretch flange corner strain across the corner axis and a stretch flange bottom center strain across the centerline axis for each stretch flange. In the preferred embodiment, this calculation is accomplished utilizing customized strain correlations for the workpiece material as developed from strain test data of workpiece samples. The calculated strains are then compared with the workpiece yield strain to determine workpiece suitability.
The method also calculates a combined stress from multiple step bends. If multiple steps are present, the method determines the location of maximum strain for each bend and adds the strains at each of the locations where a maximum strain is present. The total strain at each of the respective locations can then be compared to the material yield strain for determining the suitability of the workpiece material.
The method of the present invention can also determine the suitability of a workpiece having a shrink flange defining a corner axis and a centerline axis. If a shrink flange is present, the user inputs an arc length, a bend radius, a bend angle, a bend contour radius, a flange width, and a press forming pressure for each shrink flange, and calculates a bend strain across the corner axis at the bend line, and a bottom center strain across-the centerline axis. The strains can be calculated according to customized strain correlations developed from strain test data of workpiece samples. The straight bend strain can then be compared with the material yield strain and the bottom center strain can be compared with a material buckle strain for determining suitability of the workpiece material.
The method of the present invention can also determine spring back deformation of a workpiece material. Two types of spring back can be evaluated. First, straight bend spring back can be determined by inputting a workpiece thickness, bend angle, bend radius, and press forming pressure, and calculating the straight bend spring back deformation according to a customized straight bend spring back correlation developed from testing workpiece samples. Second, curved bend spring back can be determined by inputting a workpiece thickness, bend angle, bend radius, contour radius, and press forming pressure, and calculating the curved bend spring back deformation according to a customized curved bend spring back correlation developed from testing workpiece samples. The as-formed dimensions of the workpiece can then be adjusted for the calculated spring back to determine suitability of the workpiece and the press forming procedure.
In the above manner, the formability of a workpiece can be determined based on customized strain correlations developed for the specific material and the types of bends that will be formed. In addition, once the correlations are developed, the correlations can be used to select a workpiece for other shapes formed from the same material. Thus, testing does not have to be repeated once the correlations are developed for a particular material. Moreover, the method can be used to predict workpiece spring back for designing the workpiece and the press forming procedures.